- #1
EnSlavingBlair
- 36
- 6
Hi,
I'm currently trying to wrap my head around how the Ly-α forest works. This is what I have so far;
A distant quasar produces these Ly-α photons, which occur when a hydrogen's electron drops from n=2 to n=1, and has a wavelength of around 121nm (depending on the exact transition). This light gets redshifted as it travels due to the expansion of the Universe, and eventually it's so redshifted its wavelength has become 21cm. At this wavelength the photon can be absorbed by neutral hydrogen, and so the observed flux of the quasar at this wavelength decreases if the photons happen to be traveling through a cloud of neutral hydrogen at this redshift.
However, where it doesn't make sense to me is the fact there is a forest of these absorption lines. Surely, because the Ly-α line has a specific wavelength, it can only be absorbed by the neutral hydrogen at a specific redshift, not at multiple redshifts?
If the quasar was more like a black body, where it emitted all wavelengths, then this forest would make more sense, as the lower energy light would redshift to the 21cm line sooner than the more energetic photons, and I can see how that would create a forest effect. I cannot wrap my head around this idea if we're only looking at the Ly-α lines that are coming from the quasar.
I will be very grateful for an explanation of what concept I'm missing here.
Cheers,
EnSlavingBlair
I'm currently trying to wrap my head around how the Ly-α forest works. This is what I have so far;
A distant quasar produces these Ly-α photons, which occur when a hydrogen's electron drops from n=2 to n=1, and has a wavelength of around 121nm (depending on the exact transition). This light gets redshifted as it travels due to the expansion of the Universe, and eventually it's so redshifted its wavelength has become 21cm. At this wavelength the photon can be absorbed by neutral hydrogen, and so the observed flux of the quasar at this wavelength decreases if the photons happen to be traveling through a cloud of neutral hydrogen at this redshift.
However, where it doesn't make sense to me is the fact there is a forest of these absorption lines. Surely, because the Ly-α line has a specific wavelength, it can only be absorbed by the neutral hydrogen at a specific redshift, not at multiple redshifts?
If the quasar was more like a black body, where it emitted all wavelengths, then this forest would make more sense, as the lower energy light would redshift to the 21cm line sooner than the more energetic photons, and I can see how that would create a forest effect. I cannot wrap my head around this idea if we're only looking at the Ly-α lines that are coming from the quasar.
I will be very grateful for an explanation of what concept I'm missing here.
Cheers,
EnSlavingBlair